Slippage of a chain or a belt in a continuously variable transmission (CVT) can be detected by sensing axial displacement of a pulley with a position sensor. For instance, a transmission gear ratio determined from the width of a drive pulley detected by a position sensor through arithmetic operation is compared with an actual transmission gear ratio computed from the number of rotations of each of drive pulleys and driven pulleys, and occurrence of slippage can be detected on the basis of whether or not a difference between the ratios is equal to a predetermined value or greater (see; for instance, JP-A-2005-28915). Slippage can also be controlled by adjusting clamping force of a pulley.
Proposed in Japanese Patent No. 2606350 is an apparatus serving as a transmission of a related-art belt-type CVT.
In the apparatus of this type, a pair of hydraulic actuators are provided for changing an effective diameter of a primary pulley and that of a secondary pulley. There is provided a control valve for controlling a pressure of the hydraulic actuator that generates clamping force for the primary pulley, and there is provided a relief valve for controlling a hydraulic pressure of the hydraulic actuator that generates clamping force for the secondary pulley.